This invention relates to tank volatile liquid level gauges, and, more particularly, to level gauges for tanks containing liquids, such as propane, that are stored under pressure, in part in liquid state and in part in gaseous state, and released in gaseous state for use.
Many a backyard cookout has been delayed or ruined when the barbecue grill propane tank has suddenly run dry due to the inability of available tank gauges to provide adequate, timely warning of low tank volume. Propane, as well other similar volatile materials, is stored, e.g., in tanks, under pressure. Typically, when the tank is full, or near full, and upright, a portion of the material contents (in the lower section of the tank) is in liquid state and a portion of the material contents (in the upper section of the tank, adjacent the outlet) is in gaseous state. The ratio of liquid-to-gas depends on a combination of: the mass of material within the tank, the tank volume, the temperature of the contents, and the pressure within the tank. As material is released from the tank in gaseous state for use, the resulting decrease in material mass and lowering of pressure within the tank causes a portion of the liquid contents to change to gaseous state and expand. As a result, the pressure within the tank remains relatively constant until all of the liquid material changes to gas. At that point, with no remaining reservoir of liquid, removal of further gas causes the pressure to drop precipitously, and the tank is soon empty. As a consequence, it has been found difficult to provide timely warning of a near-empty tank by use of a pressure gauge, in particular, for smaller portable propane tanks.
Others have proposed to address this problem by constructing barbecue grills in which the tank rests upon a crude scale or balance, with the object of providing an indication of material mass within the tank, or at least a warning when that mass decreases below a minimum level. Still others have sought to provide an indication of tank liquid level using adhesive strips containing a thermally sensitive medium, e.g., liquid crystals. The strips are applied to the outside surface of the tank at a region of desired minimum tank volume level, e.g., one-quarter tank, and seek to indicate the liquid level by sensing the location of its liquid-to-gas interface by the marked temperature difference caused by change of state from liquid to gas (with attendant consumption of calories and resulting lowering of the local temperature).
In other applications, it has been known to provide tank gauges that employ a float and coupled magnets for indication of liquid level or volume. It has also been known to provide tank closure valves actuated by a float at a predetermined liquid level or volume, e.g., as described in Ballard et al. U.S. Pat. No. 768,109 and Moore U.S. Pat. No. 3,283,578.
The invention thus provides a tank liquid level or volume gauge for use with volatile materials, such as propane, which are stored in a tank in liquid form under pressure and released from the tank for use in gaseous form. The liquid level or volume gauge, operating without dynamic seals, employs a float with a float magnet that couples with a liquid level indication magnet across a fluid impermeable wall to indicate the tank liquid level or volume. The gauge may also include a dial or other device that converts the indication of liquid level into an indication of liquid volume.
In another embodiment, the invention provides a tank liquid level or volume gauge as described above that also signals to a delivery pump to shut off flow when the tank contains a predetermined volume of fluid. In particular, when the liquid in the tank reaches a predetermined level, the float actuates an inlet closure valve to increase inlet flow resistance, creating a shutoff signal.
In one aspect, the invention features a tank liquid level or volume gauge for use with a tank defining a volume containing volatile materials that are stored as a liquid under pressure and released from the tank for use in gaseous form, including: (a) a float member extending into the tank, the float member defining a float chamber configured to allow the float member to float in the liquid; (b) a float magnet joined to an upper portion of the float member; (c) an elongated shaft positioned at an upper end of the tank, the upper portion of the float member being telescopically engaged with the elongated shaft; (d) a fluid impermeable, non-magnetic wall disposed between the upper portion of the float member and the elongated shaft; and (e) a liquid level indication magnet positioned to couple with the float magnet across the fluid impermeable wall for axial positioning of the shaft in response to axial positioning of the float member, the shaft member being configured to indicate the tank liquid level by its axial positioning.
Some implementations include one or more of the following features. The liquid level or volume gauge further includes a tank upon which the gauge is mounted. The gauge further includes a coupling converting generally axial movement of the float member into lateral or rotational movement. The gauge further includes a shut-off device signaling to a delivery pump to shut off flow when the tank contains a predetermined volume of fluid. When the liquid in the tank reaches a predetermined level, a shutoff signal is created by the float member actuating an inlet closure valve to increase inlet flow resistance. The gauge further includes a cover for the tank, the cover having an indicator region through which an upper portion of the shaft is visible to a user of the tank. The upper portion of the shaft carries an indicator scale configured to provide a visual indication of the liquid level. The indicator scale includes a member having indicator panels of different colors, the colors being arranged to be displayed sequentially through the indicator region of the cover. The indicator region includes a window. The window is lens or dome-shaped. The indicator panels are green, yellow and red, to indicate, respectively, that the tank contains an adequate level of liquid, that the level of liquid has reached a low level, and that the tank is almost empty. The gauge is configured so that the green indicator panel indicates that there is sufficient liquid for at least 6 hours of operation, the yellow indicator panel indicates that there is sufficient liquid for about 2-6 hours of operation, and the red indicator panel indicates that there is sufficient liquid for 2 hours of operation or less. The volatile material is propane. The float member includes an elongated tube that extends sufficiently far into the tank so as to float when the liquid has reached a relatively low level.